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PTP Precision Time Protocol

The PTP Precision Time Protocol is used for synchronisation of several cameras
in an Ethernet network.

The Precision Time Protocol (PTP) IEEE 1588 enables the exact cycle
synchronisation of several devices in an Ethernet system. When the clocks of
the devices such as cameras, PCs and sensors, are synchronised, future
software image triggers can be synchronised with 2µs. The GigE Vision 2.0
standard has incorporated PTP IEEE 1588 and warrants maximum compatibility
between machine vision hardware and software suppliers in future.

PTP offers precision in the microseconds range and - as far as precision is
concerned - is positioned in-between the competing standards Network Time
Protocol (NTP, a few milliseconds) and Global Positioning System (GPS,
nanoseconds). Component cost is comparatively low. PTP is defined in IEEE 1588
and has developed into a widely known network standard with a growing
community of supported hardware and
software suppliers.

Synchronisation of several cameras

Cameras supporting PTP are put in a special PTP mode which manages and defines
the synchronisation of the camera clocks. The device clock of the master is
output with the timestamp of the camera and synchronises several cameras. When
the PTP mode is set to 'master', 'slave' or 'auto', the camera synchronisation
starts in the network provided one device is configured as master.

How is PTP synchronisation obtained?

The process starts with the "master" camera which sends a "sync" message via
multicast messaging. The "slave" camera calculates the time difference between
its internal cycle and that of the master camera. In addition, Delay Request
Message (by the slave) and Delay Response Message (by the master) are sent to
synchronise the slave clock with the master cycle. When the time difference of
the clock is 2µs, the cameras are in synchronous mode. In addition to the
exchange of synchronisation messages, the clocks are constantly readjusted by
filters and statistical methods in order to eliminate deviations due to the
physical layers, the network, repeater or switch.

Planned software trigger with exact synchronisation

When all device clocks are synchronised, it is either possible to obtain image
sequences from all cameras at a previously defined image frequency with exact
synchronisation, or to capture future individual events in synchronous manner
by simultaneous software trigger. In contrast with asynchronous triggering,
synchronous triggering requires knowledge of the future trigger time. By using
a GPS timer, multiple camera systems can be synchronised to "real-world time".
In this set-up every camera in a system is synchronised with the GPS master
which in turn is synchronised to the "real-world time" using GPS.